1. Field of the Invention
The present invention relates to a system for controlling data communication between a plurality of terminal units containing information and a main control assembly connected to the terminal units by transmission lines for controlling the terminal units. A number of the terminal units are controlled in a predetermined sequence in accordance with the main program of the main control assembly. The other terminal units are not included in the sequential processing under the main program.
2. Description of the Prior Art
With systems comprising a plurality of terminal units and a main control unit connected to the terminal units by transmission lines and having a central processor for controlling the terminal units, a mode of communication control is already known in which the main control unit transmits to the terminal units a message containing a terminal unit designating address, and one of the terminal units whose address is identical with the address of the received message transmits a response message to the main control unit. When the terminal unit has the data to be sent to the main control unit, the terminal unit transmits the data to the main control unit in response to the message from the control unit. With the conventional method of communication control, however, the exchange of messages between the main control unit and the terminal units is conducted entirely independently of (non-synchronously with) the main program of the main control unit comprising a series of instructions for the terminal units. Accordingly the main control unit is unable to perform data communication only with the terminal unit desired for executing the work of a particular step of the main program. The conventional method therefore has the drawback of being low in transmission line efficiency and having poor work performing speed.
It appears useful to effect communication control in corresponding relation to the main program so that for each step of the main program of the main control unit, the main control unit performs data communication only with the terminal unit which is needed for performing the work of the step. Such communication control is desirable when the terminal units are controllable in a predetermined sequence according to the main program of the main control unit. However, if the terminal units include a terminal unit which does not participate in the sequential processing under the main program of the main control unit, difficulties arise in the data communication between that terminal unit and the main control unit.
On the other hand, with systems comprising a main control unit and terminal units therefor, one must monitor or watch the other to check for troubles or malfunctions. When the terminal units are adapted to watch the main control unit in the prior art, each of the terminal units checks whether or not the message addressed to the terminal unit by the main control unit is transmitted within a specified period of time to detect the possible malfunction. Each terminal unit is therefore provided with a monitoring timer and resets the timer when the address of the received message is identical with its own address, and detects a malfunction of the main control unit when the time set on the timer is over and no such message is received.
This method of monitoring nevertheless heavily burdens the main control unit since the unit must poll some message to all the terminal units within the predetermined period of time. The larger the number of the terminal units, the greater is the burden on the main control unit. The main control unit may have to poll the terminal units merely to reset their monitoring timers. This leads to a reduced line efficiency.
When the power for the terminal unit is restored after a failure, or when the terminal unit has detected a malfunction in the main control unit, the terminal unit notifies the main unit of the malfunction in the following manner.
(1) The terminal transmits to the main unit a message indicating the power after the restoration or to the effect that a malfunction has been detected. PA1 (2) The terminal unit remains non-responsive when the power is restored, for at least such a period of time as to enable the main unit to judge that the terminal is not responsive to a message from the main unit.
However, the first method requires a program for the main unit to analyze the message from the terminal unit, while such a program is complex. With the second method, the terminal unit remains non-responsive for an unduly prolonged period of time despite the restoration of power, causing a waste of time.